In a typical cellular radio system, radio or wireless terminals (also known as terminal devices, mobile devices, mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (in a Universal Mobile Telecommunications System (UMTS) network) or “eNodeB” (in a Long Term Evolution (LTE) network). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UEs) within range of the base stations.
In some radio access networks, several base stations may be connected (e.g., by landlines or microwave) to a radio network controller (RNC) or a base station controller (BSC). The radio network controller supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access (WCDMA) for UEs.
In the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third and subsequent generation networks, and UTRAN specifically, and investigate enhanced data rate and radio capacity. 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies. A number of releases for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification have issued, and as with most specifications, the standard is likely to evolve further. E-UTRAN comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
LTE is a variant of a 3GPP radio access technology where the radio base station nodes are connected to a core network (via Access Gateways (AGWs)) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeBs in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has what is sometimes termed a “flat” architecture including radio base station nodes without reporting to radio network controller (RNC) nodes.
A currently popular vision of the future of cellular networks includes terminal devices in the form of machines or other autonomous devices communicating between each other (or with an application server) without human interaction. A typical scenario is to have sensors sending measurements infrequently, where each of the transmissions would consist of only small amounts of data. This type of communication is called machine-to-machine (M2M) communication in the literature, or machine-type communication (MTC), in 3GPP.
UEs in cellular systems (such as 3GPP WCDMA, LTE) are most commonly battery driven and the power consumption of these devices is therefore an important factor.
In the context of MTC, many of the devices are expected to be battery operated as well. Sensors and other devices may reside in remote locations and the number of deployed devices could be so large that it would be practically infeasible to replace or frequently recharge the batteries in these kinds of devices. Thus, it is an important goal to aim for reduction in the power consumption when considering improvements for current cellular systems.
An existing means to reduce the battery power consumption is to use discontinuous reception (DRX), a feature in which the UE's receiver is switched off except at configured intervals.
Currently the longest specified DRX cycle lengths are 2.56 seconds and 5.12 seconds for EUTRA and UTRA, respectively. However, it would be beneficial to extend the DRX cycle lengths beyond currently specified values to further reduce the battery power consumption, especially for the benefit of MTC devices where there is no possibility for interactive charging of the battery on a regular basis. Although longer DRX cycle lengths naturally cause larger delays in the downlink, this is typically not a problem for delay insensitive traffic such as that generated by MTC devices.
However, the DRX cycle length is currently limited by a System Frame Number (SFN) period. The SFN is used by UEs to keep synchronisation with the network and is used as a timing reference. In LTE the SFN period is 1024 radio frames equal to 10.24 seconds and in High-Speed Packet Access (HSPA) the SFN period is 4096 radio frames equal to 40.96 seconds.
In LTE a UE needs 10 bits to determine the SFN since it takes 1024 different values. Eight of these bits are broadcast by the network in a system frame number field in the master information block (MIB). The MIB is broadcast for 40 ms during which the same information (including the value in the system frame number field) is repeated four times, i.e. every 10 ms. As the MIB only carries eight of the bits for the SFN, the last two bits, which gives four values for the SFN within the 40 ms period, are retrieved implicitly by the UE from the different scrambling codes used for the four copies of the MIB broadcast in each 10 ms period.